Document & expose printing of custom AbstractTestSet (#53215)

This was previously alluded to in the docs as being possible ("just
record if we're not the top-level parent"), but trying to do that didn't
actually do anything because the printing in the `DefaultTestSet`
ignored anything that wasn't a `Test.Result` or `Test.DefaultTestSet`.
This is particularly problematic if any of the child-testsets had
failures, because those failures would never be shown to a user.

Co-authored-by: Sukera <[email protected]>
Co-authored-by: Ian Butterworth <[email protected]>

stdlib/Test/docs/src/index.md

@@ -322,8 +322,12 @@ function finish(ts::CustomTestSet)
     # just record if we're not the top-level parent
     if get_testset_depth() > 0
         record(get_testset(), ts)
+        return ts
     end
-    ts
+
+    # so the results are printed if we are at the top level
+    Test.print_test_results(ts)
+    return ts
 end
 ```
 
@@ -338,6 +342,45 @@ And using that testset looks like:
 end
 ```
 
+In order to use a custom testset and have the recorded results printed as part of any outer default testset,
+also define `Test.get_test_counts`. This might look like so:
+
+```julia
+using Test: AbstractTestSet, Pass, Fail, Error, Broken, get_test_counts, TestCounts, format_duration
+
+function Test.get_test_counts(ts::CustomTestSet)
+    passes, fails, errors, broken = 0, 0, 0, 0
+    # cumulative results
+    c_passes, c_fails, c_errors, c_broken = 0, 0, 0, 0
+
+    for t in ts.results
+        # count up results
+        isa(t, Pass)   && (passes += 1)
+        isa(t, Fail)   && (fails  += 1)
+        isa(t, Error)  && (errors += 1)
+        isa(t, Broken) && (broken += 1)
+        # handle children
+        if isa(t, AbstractTestSet)
+            tc = get_test_counts(t)::TestCounts
+            c_passes += tc.passes + tc.cumulative_passes
+            c_fails  += tc.fails + tc.cumulative_fails
+            c_errors += tc.errors + tc.cumulative_errors
+            c_broken += tc.broken + tc.cumulative_broken
+        end
+    end
+    # get a duration, if we have one
+    duration = format_duration(ts)
+    return TestCounts(true, passes, fails, errors, broken, c_passes, c_fails, c_errors, c_broken, duration)
+end
+```
+
+```@docs
+Test.TestCounts
+Test.get_test_counts
+Test.format_duration
+Test.print_test_results
+```
+
 ## Test utilities
 
 ```@docs